The SOA lookup for edns512 could succeed if the negative response
for ns.edns512/AAAA completed before all the edns512/SOA query
attempts are made.  The ns.edns512/AAAA lookup returns tc=1 and
the SOA record is cached after processing the NODATA response.
Lookup a TXT record at edns512 and look it up instead of the
SOA record.

Removed 'checking that TCP failures do not influence EDNS statistics
in the ADB' as it is no longer appropriate.